One fascinating aspect of human cognition is that it seems to have a specialized mechanism devoted to the recognition of human faces. This mechanism is demonstrated most dramatically by a condition called prosopagnosia, or face-blindness. Victims of prosopagnosia have difficulty recognizing human faces, but little or no difficulty with other visual recognition tasks. Prosopagniacs often compensate using other recognition techniques. The music professor Dr. P with prosopagnosia in Oliver Sack's The Man Who Mistook his Wife for a Hat used voice recognition, among other techniques.

Prosopagnosia was originally considered to be very rare, but CogNews notes some recent research by Ken Nakayama and Richard Russell that claims that prosopagnosia is much more widespread, with perhaps 1 in 50 people suffering from the disorder. Unfortunately, I can't find the published research itself online (they may be writing it up), since it would be good to understand the level of impairment that they examined. Their press release indicates close-cropped black-and-white photos of faces were used.

One fascinating part of the researchers' main web site, www.faceblind.org, is its list of personal web pages of prosopagnosiacs, who talk about how they deal with their disability. One prosopagnosiac, Bill Choisser, has written an online book about his experiences with prosopagnosia that is fascinating to read. For example:

"For me, I feel a very definite difference when my eyes drift upward across the jaw line. Below that line, what I see can evoke an emotion of attachment. Above it, I feel mainly emptiness, and the shape of hair lines is all I can remember easily. Two short-haired guys with beardless faces will look exactly alike. I can no more tell them apart than I can two German shepherds. The only exception, as I said above, is for people whom I've seen many many times." (Choisser, Chapter 8A)

These kinds of video-based changes are quite common, and there are even people who watch for them in popular television series. See, for example, this site for Star Trek and X-Files includes such changes under "continuity and production problems."